The Melancholy of Suzumiya Haruhi

In one of the episodes of the Melancholy of Suzumiya Haruhi, Yuki and Haruhi acted as stand-ins for four-member band in their school cultural festival. Surprisingly (or unsurprisingly), Yuki was able to memorize the music piece just by looking at it, and played the guitar flawlessly on stage. Kyon later commented that Yuki probably played along with Haruhi to keep her from being bored.

Awesome cosplay! I had to crop this, but the original image is linked on the thumbnail, so feel free to see the high-res version! Thanks to Shinji for sending this in!